Regeneration of flue gas desulfurization sorbents

ABSTRACT

SOLID SORBENTS FOR THE REMOVAL OF SULFUR DIOXIDE FROM GASES, SUCH AS COPPER OXIDE ON A SUITABLE CARRIER MATERIAL SUCH AS ALUMINA, ARE REGENERATED WITH A REGENERATION GAS COMPRISING HYDROGEN AND ABOUT 50 TO 95% BY VOLUME OF STEAM. THE PRESENCE OF STEAM IN THE AMOUNTS INDICATED REDUCES THE AMOUNT OF SULFIDE FORMATION AND THEREBY INCREASES THE CAPACITY OF THE SORBENT AND IMPROVES THE UTILIZATION OF REDUCING GAS.

United States Patent 3,778,501 REGENERATION 0F FLUE GAS DESULFA- TIONSORBENTS Robert J. Lang, Watchung, N.J., Eugene L. Holt, Elmhurst, N.Y.,and David N. Stonebaclk, Westfield, N..l., assignors to Esso Researchand Engineering Company No Drawing. Continuation-impart of abandonedapplication Ser. No. 790,844, Jan. 13, 1969. This application June 4,1971, Ser. No. 150,173

Int. Cl. B01d 53/34 US. Cl. 423-244 Claims ABSTRACT OF TIE DISCLOSURESolid sorbents for the removal of sulfur dioxide from gases, such ascopper oxide on a suitable carrier material such as alumina, areregenerated with a regeneration gas comprising hydrogen and about 50 to95% by volume of steam. The presence of steam in the amounts indicatedreduces the amount of sulfide formation and thereby increases thecapacity of the sorbent and improves the utilization of reducing gas.

CROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of our copending application Ser. No. 790,844,filed Jan. 13, 1969, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to processes forremoval of sulfur dioxide from gases such as flue gas, and moreparticularly to processes for the regeneration of solid sorbents used inthe removal of sulfur dioxide.

Some high sulfur coals and fuel oils which are burned in theboiler-generator units of electrical power stations and other industrialfurnaces are the source of air pollution problems in heavily populatedareas. The flue gas from these fuels contains S0 and S0 and efforts arebeing made to develop eflicient processes for removing these materials.While the problem is generally greater for electric power plants, it isalso a problem for other industries where heat must be generated in aboiler, in ore smelting, and in other situations in which gasescontaining sulfur dioxide are produced.

It is known that sulfur dioxide can be removed from flue gas bycontacting the flue gas with a suitable solid sorbent. A preferredsorbent for the removal of sulfur dioxide from flue gas is a sorbentcomprising copper, copper oxide or mixture thereof supported on alumina.Flue gas desulfurization processes using copper on alumina sorbents aredescribed, for example, in British Patent No. 1,089,716 and in US. Pat.No. 3,501,897.

The copper on alumina sorbent is used in a cyclic regenerative processin which the 80;, content of the flue gas reacts with the copper oxideof the sorbent in one step, followed by regeneration of the sorbent withsimultaneous desorption of S0 Flue gas from a combustion source, whichordinarily contains both sulfur dioxide and oxygen, is contacted with abed of the solid sorbent during the desulfurization step of the cycle.The copper is oxidized quantitatively to CuO, which in turn is partiallysulfated to copper sulfate. Equations 1 and 2 below are representativeof the reactions taking place during this step of the cycle.

When the sulfur dioxide concentration in the exit gas exceeds apredetermined amount, the sorbent bed is taken off stream andregenerated by passage of a reducing gas in contact with the bed.Reducing gases known in the art include hydrogen, carbon monoxide, andmixtures thereof, and low molecular weight hydrocarbons such as methane,ethane, propane, butane, and natural gas. Regeneration takes place atapproximately the same temperature as desulfurization. The regenerationotf gas contains S0 in concentrations substantially higher than theconoentration of S0 in the flue gas. This sulfur dioxide can be Workedup to either sulfur or sulfuric acid. Simultaneously with the desorptionof sulfur dioxide, the sorbent is regenerated with the decomposition ofcopper sulfate into copper oxide, metallic copper, copper sulfide, ormixtures thereof, as shown by Equations 3, 4 and Hydrogen is the mostactive of the reducing gases enumerated above. It is highly desirablefor its high activity an because, unlike the hydrocarbons, it does notcause the deposition of coke on the sorbent. Carbon monoxide is alsoactive, although less so than hydrogen, and is also desirable because ofthe absence of coke deposition on the sorbent. However, one difiicultyencountered in regeneration with hydrogen, carbon monoxide, or mixturesthereof, is that part of the copper sulfate is reduced to copper sulfideas shown by Equation 5, rather than to either copper oxide or metalliccopper as shown by Equations 3 and 4. Formation of copper sulfide isundesirable for two reasons. First, a much larger quantity of reducinggas is required to reduce the copper sulfate to copper sulfide than isrequired to reduce copper sulfate to metallic copper. Addition ofEquations 3 and 4 above shows that two moles of hydrogen are required toreduce copper sulfate to metallic copper, while Equation 5 shows thatfour moles of hydrogen are consumed in reducing copper sulfate to cupricsulfide. Secondly, copper sulfide has very little capacity for removingsulfur dioxide from flue gas. On a subsequent flue gas desulfurizationcycle, copper sulfide is oxdized to copper sulfate according to Equation6 below.

Copper sulfide has been illustrated as cupric sulfide in the aboveequations, although the sulfide formed on regeneration is probably amixture of cuprous and cupric sulfides, and possibly a mixture in whichcuprous sulfide predominates.

SUMMARY OF THE INVENTION According to the present invention, it has beendis covered that sulfide formation during the regeneration of flue gasdesulfurization sorbents can be considerably diminished and theutilization of reducing gas markedly improved by using as theregeneration gas a mixture comprising a reducing gas and about 50 toabout by volume of steam. The reducing gas is either hydrogen, carbonmonoxide, or a mixture thereof.

DETAILED DESCRIPTION OF THE INVENTION In a complete operating cycleemploying the regeneration procedure of the present invention, sulfurdioxide is removed from a gas mixture containing both sulfur dioxide andoxygen, such as flue gas, by passing the gas mixture in contact with asolid sorbent comprising a heavy metal, heavy metal oxide, or mixturethereof and a solid carrier material until sulfur dioxide breaks throughinto the exit gas. The heavy metal or metal oxide is at least partiallysulfated in the process. The sorbent is then regenerated according tothis invention by contacting the sorbent with a regeneration gascomprising a mixture of hydrogen, CO, or a mixture thereof, and about 50to about 95% by volume of steam, as will be described in greater detailbelow. Sulfur dioxide is desorbed during regeneration. The concentrationof sulfur dioxide in the regeneration off gas is substantially greaterthan the sulfur dioxide concentration in the incoming flue gas.

Sorbents comprising copper, copper oxide, or a mixture of the two onalumina are preferred in the practice of this invention. The copper onalumina sorbent used in the present invention may be of a type known inthe art. Such a sorbent initially contains copper, copper oxide, or amixture thereof supported on alumina or a high alumina material. Thesorbents described in British Patent No. 1,089,716 are examples ofsuitable sorbents. Any adsorbent grade alumina or material of highalumina content may be used as the carrier according to this invention.Suitable carriers include natural clays, pretreated or not with acid,bauxite, synthetic alumina, and aluminasilica mixtures of high aluminacontent. The sorbent grade aluminas have a high surface area, usuallyover about 100 square meters per gram.

The sorbent can be prepared by known techniques. According to onemethod, the carrier material is impregnated with an aqueous solution ofa copper salt and is then dried or calcined. The sorbent aftercalcination is in the form of copper oxide on the carrier. The copperoxide may be reduced to metallic copper if desired. Another method ofsorbent preparation is to mix a copper compound in a carrier intimatelythrough co-precipitation, with subsequent drying and calcining. Thecopper content of the sorbent may vary within wide limits. As a rule, itis at least 1% by weight, and preferably no more than about 25% byweight, of the sorbent, preferably about 2 to about 12% by weight of thesorbent (i.e., the Weight of copper plus the weight of carrier).

The sorbent may be prepared in any desired physical form, such aspellets, extrudates, etc. In the case of pellets, the carrier may bepreformed into the desired shape and then impregnated With the coppersalt.

Regeneration is carried out according to the present invention bycontacting the spent solid sorbent with a regeneration gas comprising(1) a reducing gas selected from the group consisting of hydrogen,carbon monoxide, and mixtures thereof, and (2) about 50 to about 95% byvolume of steam. Applicants have discovered that sulfide formation ismaterially reduced and the capacity of the copper sorbent for removingsulfur dioxide from flue gas is materially increased by using a gasmixture which contains at least 50% by volume of steam for regeneration.As the percentage of steam in the regeneration gas is increased, theamount of sulfide formation is reduced. However, it is not desirable foreconomic considerations to use regeneration gases containing more thanabout 95% by volume steam, because of the excessive amount of gasrequired for regeneration when the volume percentage of steam exceedsthis amount. Preferred regeneration gas mixtures contain from about 60%to about 85% by volume of steam. Gas mixtures containing less than about60% by volume of steam are less desirable because of the greater sulfideformation. Gas mixtures containing more than about 85% by volume ofsteam are not optimum because of the large volume of regeneration gasrequired, even though sulfide formation is actually less than whenregeneration gas mixtures containing 60 to 85 by volume steam are used.

The preferred reducing gas component in regeneration gases according tothis invention is hydrogen. It is desirable for the volume percentage ofhydrogen to be greater than the volume percentage of CO in a mixturecontaining both gases, since hydrogen has greater activity Regenerationgas according to this invention may also contain some inert gases, suchas carbon dioxide. Carbon dioxide is normally present simply because ofthe process by which the regeneration gas is preferably made.

Preferred regeneration temperatures are in the range of about 600 to 900R, which is about the same as the preferred flue gas desulfurizationtemperature. By conducting both desulfurization and regeneration atabout the same temperatures, thermal shock, which materially shortensthe life of the sorbent, is minimized.

The regeneration off gas contains sulfur dioxide, steam, unreactedhydrogen and/ or carbon monoxide, plus other constituents, e.g., carbondioxide, present in the regeneration gas. The off gas can be treated toconvert the sulfur dioxide therein to sulfur or sulfuric acid.

The effectiveness of mixtures comprising hydrogen and steam forregeneration of sorbents cannot be explained strictly in terms ofhydrogen concentration. While hydrogen-steam mixtures containing about50 to by volume and conversely about 5 to 50% by volume of hydrogen havebeen found highly effective in preventing sulfide formation,hydrogen-nitrogen mixtures of equal volume percentage hydrogen are lesseffective.

Regeneration gas for the present invention can be made by catalyticallyreforming methane with steam according to methods known in the art. Thereformer etfiuent contains steam, hydrogen, carbon monoxide, carbondioxide, and usually trace amounts of methane. The amounts of stream inthe reformer effluent may be as much as about 40% by volume. Since alarger amount of steam is rc quired in the regeneration of gas of thepresent invention, it is necessary either 0 add steam to the effiuentgas or to use a greater than normal excess of steam in the catalyticconverter. The former, of course, is preferable. The addition of steamto the reformer efiiuent not only brings the amount of steam in the gasup to that desired, but also is effective in reducing the gastemperature below the reformer effluent temperature, which is about 1500F. When a regeneration gas substantially free of carbon monoxide, orhaving a higher ratio of H to CO than the reformer efi luent, isdesired, the amount of hydrogen may be increased and the amount of COcorrespondingly decreased by conventional water gas shift techniques.This represents a preferred mode of operation. The regeneration gas maycontain trace amounts of methane which pass through the catalyticreformer unreacted. The Water gas shift efl'luent, which contains over50% by volume of steam, is adjusted to the desired regeneration gasinlet temperature of about 600 to about 900 F. as necessary and ispassed without removal of any constituents therefrom to the flue gasdesulfurization reactor.

Typical results of operations in accordance with the process of thisinvention are given in the following examples. These examples are merelyillustrative and not limitative. In each example, a copper on aluminasorbent was oxidized and partially sulfated with simulated flue gascontaining about 0.3% by volume of S0 about 0.8% by volume of oxygen,13% by volume of C0 balance nitrogen (percentages on dry basis), andsaturated with water at room temperature. The percentage of coppersulfated (i.e., converted to copper sulfate) in each run is given. Thenthe sorbents were regenerated with gas mixtures of various compositionsas indicated in the tables. These gas mixtures include hydrogen-steamand carbon monoxide-steam regeneration gases according to thisinvention, as well as other gas mixtures given for the purpose ofcomparison. Regeneration conditions, i.e., gas inlet temperature andspace velocity, are given in each example.

In each example, the number of moles of S0 desorbed per mole of reducinggas (H or CO as the case may be) reacted, is given in the tables.Examples 1-4, describing the use of hydrogen as the reducing gas, alsogive the theoretical maximum ratio of moles of S0 desorbed per mole ofhydrogen reacted, based on the assumption that hydrogen willquantitatively reduce any CuO present according to Equation 4. Themaximum theoretical SO2/H2 mole ratio, based on complete sulfation ofthe sorbent, is 0.50. The theoretical S0 /H mole ratio is lower than0.50 when the sorbent is only partially sulfated. Also given is thequotient of the actual SO /H mole ratio. This quotient is a good measureof the extent of sulfide formation.

EXAMPLE 1 A sorbent consisting of to 12% by weight copper on alumina wasused to remove sulfur dioxide from flue gas, and the sorbent wasregenerated by contacting it with various hydrogen-steam mixtures orwith dry hydrogen. Results are indicated in Table I below.

TABLE I Regeneration Gas rate, Mols S0 /mol H, Percent v./v./hr. Q of CuTemp., percent Theotient, Run No. sulfated F H; Steam steam reticalActual percent The above data show that excellent conversions of coppersulfate in the spent sorbent to copper, with little or no sulfideformation, were obtained in Run Nos. 1 to 4 using a regeneration gascontaining 6% hydrogen and 94% steam at a hydrogen space velocity of 500v./v./hr. These excellent conversions are indicated in the columnheading Quotient, since this heading represents the percentage ofhydrogen which is actually consumed in reducing copper sulfate or copperoxide to metallic copper (based on the assumption that all copper oxidewhich is either present at the start of the regeneration cycle or formedduring the regeneration cycle by copper sulfate decomposition is reducedto metallic copper) divided by the total quantity of hydrogen reacted.In Run No. 5, the quotient of 80% indicates that 80% of the coppersulfate undergoing reaction is converted back to metallic copper and 20%is converted to copper sulfide. Run Nos. 1-5 were conducted inaccordance with the present invention. Whether the slightly poorershowing in Run No. 5 as compared to Run Nos. l-4 is due to the lowerpercentage of steam or the higher hydrogen space velocity cannot beascertained based on these data. Comparison Run Nos. 6 and 7 show amarkedly poorer hydrogen utilization, whether based on the quantity ofsulfur dioxide liberated or on the percentage of copper sulfate reducedto metallic copper.

EXAMPLE 2 A sorbent consisting of 8.4% by Weight copper on alumina wassulfated to a high degree and then regenerated with mixtures of hydrogenand steam in various amounts. Run No. 1 is a comparison run, and RunNos. 2 and 3 were carried out according to this invention. The followingresults were obtained shown in Table II.

vention using as the regeneration gas a mixture of 25% by volumehydrogen and 75% by volume steam for 8 minutes at a total space velocityof 2000 v./v./hr. (hydrogen space velocity of 500 v./v./hr.). Resultsare shown in Table I11 below.

TABLE III Regeneration Mols SOz/mol H2 Percent Run of Cu Temp., SpaceTheo- Quotient, No. sulfated F. Gas velocity retieel Actual percent1"-.- 49 600 A 3,300 0.33 0.23 2- 56 600 A 3, 300 0. 36 0. 25 70 3- 44600 B 2, 000 0. 31 0.21 95 4- 44 600 B 2, 000 0. 31 0. 33 108 N oTE.A=87% H2, 5% CO, 3% H20, 55% N2 (all percent by volume), 14] mini,i'i,300 v./v./hr. total gas; B=25% Hz, E20, 8 min., 2,000 v./v./

r. o a gas.

EXAMPLE 4 A sorbent similar to that used in Example 1, containing 10-12%by weight of copper on alumina, was sul- TABLE II Regeneration Gas rate,Mols SO /mol H: Percent v./v./hr. Quo of Cu Temp, percent Theotient, RunNo. sulfated F. H; Steam steam retical Actual percent EXAMPLE 3 fatedwith simulated flue gas to the extent indicated in A series of runs wascarried out in order to show the effect of hydrogen-nitrogen mixturesversus hydrogen steam mixtures on reducing gas utilization duringregeneration. The sorbent was a 10-12% copper on alumina Table IV below.The sorbent was then regenerated with carbon monoxide, with and withoutadded steam, in two separate runs. The carbon monoxide space velocitywas 75 the same in both runs, and the total space velocity was muchgreater in the run where steam was used. Results are shown in Table IVbelow.

The above results show that carbon monoxide utilization is much betterwhen there is added steam (Run N0. 2) than when pure dry carbon monoxideis used (Run No. 1).

It will be noted that the number of mores of S liberated per mole of COundergoing reaction is greater than the ratio which would be obtained ifhydrogen were used as the reducing gas. This is because carbon monoxideis a less active reducing agent than hydrogen. While hydrogenquantitatively reduces all copper oxide in the sorbent to metalliccopper, it is probable that a portion of the copper oxide which iseither present in the sorbent at the beginning of the regeneration cycleor formed during that cycle is not reduced to metallic copper whencarbon monoxide is used as the reducing agent.

What is claimed is:

1. In a process for removing sulfur dioxide from a gas mixturecontaining sulfur dioxide and oxygen in which said gas mixture iscontacted with a solid sorbent comprising copper, copper oxide or amixture thereof supported on alumina to remove sulfur dioxide from saidgas mixture and in which said sorbent is regenerated and sulfur dioxideis desorbed by contacting said sorbent with a regeneration gas, theimprovement wherein said regeneration gas comprises hydrogen, carbonmonoxide, or a mixture thereof, and about to about 95% by volume steam.

2. A process according to claim 1 in which the inlet temperature of saidregeneration gas is about 600 to about 900 F.

3. A process according to claim 1 in which the inlet temperature of saidregeneration gas is about 650 to about 750 F.

4. A process according to claim 1 in which said regeneration gascomprises hydrogen and about to about by volume of steam.

5. A process according to claim 1 in which said regeneration gas isprepared by catalytically reforming a gaseous hydrocarbon and addingsteam to the reformer eflluent gas.

References Cited UNITED STATES PATENTS 2,747,968 5/1956 Pigache 23l78 S3,411,865 11/1968 Pijpers et al 2325 3,428,575 2/ 1967 Pijpers et al.2325 3,501,897 3/1970 Van Helden et a1 2325 EARL C. THOMAS, PrimaryExaminer US. Cl. X.R.

